2 Answers
2

Can one make an objective, quantifiable distinction between
Rock-n-roll and Rhythm-n-Blues?

Short answer: No.

Long Answer:

Alan Freed's use of the term rock-n-roll in the 1950s is often considered definitive. He used the term to refer to R&B combos, black vocal groups, saxophonists, black blues singers, and white artists playing in the authentic R&B style.

Rhythm-n-Blues was early Rock-n-roll.

Early rock-n-roll was the popular, and commercial (and often "white") face of Rhythm-n-Blues.

First, try to define rock and roll. I thought it was that driving back beat, until I heard an Egyptian band that could give Bo Diddley lessons. - Harry Hepcat

Whilst Wikipedia says...

"The Fat Man" is a rhythm and blues song co-written by Fats Domino and Dave Bartholomew and recorded by Fats Domino. It is considered to be one of the first rock and roll records.

Once you get towards the end of the 1950s however both terms have started to evolve along interweaving paths into RnB and the all encompassing umbrella term of Rock music.

The term "Rhythm and Blues" or "R&B" was coined by Jerry Wexler (who went on to be a famous record company executive and producer) when he was working as a journalist at Billboard Magazine, circa 1952. (Wikipedia link). Billboard published a weekly chart displaying their estimates of the relative position of retail sales of single recordings which were also broadcast on radio stations with an African-American audience, and the chart was called "Race" (as in "racially-oriented", or perhaps "oriented to the minority race" which in the United States of America meant African-Americans). Recordings made by and marketed and sold to African-Americans had previously been called "race records". Wexler proposed the name "Rhythm and Blues" at a staff meeting of Billboard magazine when they discussed wanting to change the name of the "Race" chart to something else, and his suggestion was accepted.

As was mentioned earlier, influential radio personality Alan Freed coined the term "rock and roll" circa the mid-1950s. Freed seems to have intended the usage to refer to music made by both African-American musicians and white musicians.

Quickly, though, in popular usage in the United States of America, "rock and roll" came to refer to music of a certain style recorded by white musicians and marketed and sold to a white audience. "Rhythm and Blues" refers to music that was recorded by African-American musicians and marketed and sold to an African-American audience.

Note that the key to this is the term "marketed and sold". Both "Rock and Roll" and "Rhythm and Blues" came to be designations used by the commercial music industry to describe how the music was marketed and sold, and to whom it was marketed.

Notwithstanding, there are certainly distinct artistic characteristics to each kind of music.

Stylistically

The rhythms used in the two styles are similar. I would note that rock and roll has usually involved simple instrumentation of bass, guitar, and drums, small ensembles, and strictly diatonic chord progressions (mostly just I, IV, V7 or bVII if mixolydian mode is used). R&B, on the other hand, freely incorporates the expanded harmonic palette of jazz chords, uses larger ensembles, and can be found frequently to incorporate keyboards, horn sections, woodwinds, and string orchestras. R&B also tends toward more mid-tempo and slow-tempo pieces. The rock singing style is straightforward and narrative, whereas the R&B singing style involves florid melisma and other embellishments. It can also be noted that female singers are much more prevalent in R&B than they are in rock, where male singers strongly predominate. There is also a lot of influence of hip-hop and rap found in R&B in the last fifteen or twenty years, whereas hip-hop and rap have a much more limited influence on rock (and country) in the same time period.